Smaller Size Of Precipitates Research Articles

Mechanism of the dependence of precipitation behavior on stress level during stress-aging in an Al-Zn-Mg alloy

Elastic stress plays a crucial role in controlling the precipitation characteristics of precipitates during the artificial aging of age-hardening metal materials. This study investigates the effects of stress level on the precipitation of precipitates in an Al-Zn-Mg alloy, using a combination of small-angle X-ray scattering, transmission electron microscopy, and differential scanning calorimetry to provide a quantitative comparison of the precipitates. The results demonstrate that the hardness of stress-aged alloy almost monotonically increases with higher applied external stress. Moreover, the precipitation behavior of precipitates varies markedly under different stress conditions. The studied alloy aged under low stress levels predominantly exhibited semi-coherent η' phases characterized by higher precipitate number density (6.0 ∼ 7.0 × 1022 m−3) and smaller precipitate size (3.28 ∼ 3.66 nm) compared to stress-free aged alloy (∼1.9 × 1022 m−3, ∼5.46 nm). Under high stress levels, enhanced dislocation diffusion and aggregation promote the heterogeneous nucleation and the formation of coarse η phases. This work provides further insights into manipulating the precipitation behavior of Al-Zn-Mg alloy through applied stress field, potentially offering solutions to achieve superior comprehensive properties in these alloys.

The microstructure analysis, mechanical properties, and fracture behavior of electron beam welded C-103 niobium-based refractory alloy

This study involves investigation of metallurgical and mechanical properties of C-103 niobium alloy welded by electron beam. ICP, XRD and EDS method were used to determine the composition and phase contents of the alloy. Electron beam welding was carried out on specimens with various currents and transverse speeds. To examine macro- and micro-structural characteristics of the welded samples, stereographic, optical, electron scanning microscopy and EDS methods were used. Also, mechanical properties including micro-hardness profile and tensile strength were measured. It was observed that C-103 alloy has HfO2 precipitates with two different morphologies of spherical coarse and fine particles. Within the weld zone, primary precipitates were dissolved and new nanometer-sized ones were formed regularly throughout this zone. Morphology of the formed precipitates was mostly spherical and rarely filament-like. Due to greater number and smaller size of precipitates in weld zone, hardness of weld zone was more than HAZ and base metal zone. Tensile strength of welded sample was 317.5 MPa which is equal to 55% of tensile strength of base metal. Fracture of all welded samples occurred within weld zone which can be referred to enlargement of grains in this zone. The features presented on fracture surface of unprocessed C-103 alloy, such as cross section reduction, formation of dimples, opaque appearance of fracture surface indicate ductile nature of fracture. However, emerging of relatively smooth and sharp surface, abundant vein patterns and cleavage planes obviously indicate brittle fracture. HfO2 precipitates, as well as dissolved oxygen in the matrix, significantly affected metallurgical and mechanical properties of welded C-103 alloy samples.

Effects of Nb addition on the properties and microstructure of Cu-Ni-Si-Mg alloy

In this paper, the effects of Nb addition on the mechanical properties, electrical conductivity and microstructure of the Cu-Ni-Si-Mg alloy were investigated. The results show that, after the secondary aging at 460 °C for 120 min, the alloy with 0.2 wt% Nb exhibits the best comprehensive properties with a hardness of 245.9 HV, a tensile strength of 747.28 MPa, a breaking elongation of 8.6%, and an electrical conductivity of 47.74 %IACS. The addition of Nb element can promote the precipitation of Ni 2 Si phase. The Nb-doped alloy has a higher quantity of precipitates and a smaller size of precipitates. The Nb element promotes the transition of the Ni 2 Si phase from a coherent to an incoherent relationship with the matrix in the Cu-Ni-Si-Mg alloy. In the initial stage of precipitation, the precipitation activation energy of the alloy with 0.2 wt% Nb is 46.86% lower than that of the alloy without Nb. The addition of Nb increases the proportion of Σ3 n ( n = 1, 2, 3) grain boundary in the Cu-Ni-Si-Mg alloy. Furthermore, after the secondary aging, the Nb addition increases the recrystallized texture and recrystallized grains and decreases the cold-rolled texture and defects in the alloy. • The Nb addition improves the mechanical properties and conductivity of CuNiSiMg alloy. • The Nb addition promotes the Ni 2 Si phase from coherent to incoherent relationship. • The alloy with 0.2 wt% Nb reduces the precipitation activation energy by 46.86%. • The CuNiSiMgNb alloy has more Σ3 n ( n = 1, 2, 3) CSL boundaries.

Effect of hydrogen accumulation on θ' precipitates on the shear strength of Al-Cu alloys

We investigate the effect of accumulation of H atoms on θ' strengthening precipitates in Al-Cu alloy. A relationship is established between the H accumulation and decrease in shear strength of alloy depending on the H content in precipitates of second phase. We use the previously proposed multiscale approach combining molecular dynamics (MD), continuum modeling and discrete dislocation dynamics (DDD). H atoms are located inside precipitate and at interphase boundaries. Energetically preferable places of H accumulation near and inside θ' phase in Al-Cu alloy are revealed. It is shown that the H accumulation on θ' leads to a decrease in precipitate shear resistance and causes the change in the dislocation-precipitate interaction mechanism from the Orowan loop formation to the cutting of the inclusion. The H accumulation on precipitates causes an increase in the threshold stress for the dislocation nucleation on the inclusion. The mechanism of softening of inclusions associated with a change from looping to cutting is transferred to the DDD level. The DDD results demonstrate that H charging of Al-Cu alloy can substantially decrease its shear strength. For high strength alloys with small precipitate size and/or inter-precipitate distance and with the yield stress above about 460 MPa, the reduction of strength can reach 10–25%.

Mechanical properties, corrosion behavior and microstructure evolution of zinc and scandium co-strengthened 5xxx alloy

Nowadays, it is a challenge to improve the strength of 5xxx aluminium alloy while maintaining its distinctive properties. To address this, the Zn and Sc modified high strength 5xxx aluminium alloys with good intergranular corrosion resistance were prepared through chill casting, and the effects of minor Sc on mechanical properties, corrosion behavior and microstructure evolution of Al-5.0Mg-3.0Zn alloys were systematically investigated. The results show that the Sc containing alloy has high number density and small size of intragranular precipitates, combined with the solid solution strengthening of Sc and the Al3(Sc1–x,Zrx) second phase strengthening, the strength of Al-5.0Mg-3.0Zn-0.1Sc alloy is obviously enhanced. Meanwhile, the intergranular corrosion resistance of the Sc containing alloy is also improved due to the formation of a large number of low angle grain boundaries and the weakening of the continuity of precipitated phases along the grain boundaries.

Increasing the Mechanical Strength and Corrosion Resistance of Aluminum Alloy 7075 via Hydrostatic Extrusion and Aging.

The present study investigates the correlation between mechanical properties and resistance to corrosion of hydrostatically extruded aluminum alloy 7075. Supersaturated solid solutionized samples undergo a plastic deformation process, followed by both natural and artificial aging. Furthermore, two types of hydrostatic extrusion are applied to the samples: single-stepped and double-stepped. This process is shown to influence grain refinement and the precipitation process, resulting in changes in the electrochemical properties of the samples. Hydrostatic extrusion combined with aging is shown to cause an increase in mechanical strength ranging from 50 MPa to 135 MPa in comparison to coarse-grained sample subjected to T6 heat treatment. The highest value of tensile strength is obtained for a sample subjected to single-step hydrostatic extrusion followed by natural aging. This strength increase is caused by refinement of the microstructure, in addition to the small size and number of precipitates at the grain boundaries, which are coarsened by artificial aging. Hydrostatic extrusion is also shown to increase resistance to corrosion, with the T6-treated coarse-grained sample being most susceptible to corrosion attack.

Effect of decarburisation and nitriding on the carbon content, precipitates, microstructure and texture of Nb-bearing grain-oriented silicon steel

ABSTRACTThe decarburisation annealing processes of Nb-bearing grain-oriented silicon steel under different conditions were conducted. The effect of nitriding and decarburisation annealing time on the carbon content, precipitates, microstructure and texture of Nb-bearing grain-oriented silicon steel were investigated by the various analysis methods. The results show that the carbon content in the steel decreases as decarburisation time increasing, and decreases below 0.003% eventually. Larger number density and smaller size of precipitates were obtained in the sample treated by nitriding before decarburisation annealing process, leading to fine and uniform microstructure, which is beneficial to obtain excellent magnetic properties for Nb-bearing grain-oriented silicon steel. The main texture types of the experimental samples were γ-fibres with a peak at {111}<112 >. The nitriding before decarburisation annealing process makes little effect on texture evolution during decarburisation annealing process.

Effect of solution temperature of rejuvenation heat treatment on the stability of γ′ precipitates in Ni-base superalloy IN738LC during long-term heating

This study aims to investigate the effect of time and temperature of complete and partial solutionizing heat treatment on the coarsening of γ′ precipitates in rejuvenation treatment of the used gas turbine blade IN738LC worked for 60000 h. During long-term heating (simulated under the service conditions). After the sample preparation and rejuvenation treatment, long-term heating was carried out at 900 °C and 1000 °C for 500 to 2500 h. The SEM results showed that after rejuvenation with increasing temperature and time, γ′ precipitates coarse in all the samples and the coarse γ′ precipitates appear with spherical morphology. It was observed that the amount of secondary γ′ precipitates decreases with the increase in temperature. When the solution treatment temperature drops, the amount of coarsened γ′ precipitates has increased, which shows that the temperature rise results in a lower coarsening rate and smaller precipitate size.

Effect of Magnesium on Microstructure Refinements and Properties Enhancements in High-Strength CuNiSi Alloys

Microalloying is an effective method to improve the comprehensive properties of copper alloys. The effects of magnesium on the microstructure, mechanical properties and anti-stress relaxation properties of CuNiSi alloys have been investigated. Results demonstrated that magnesium plays significant roles in refining the dendritic microstructure of the as-cast ingot, accelerating the precipitation decomposition, improving the mechanical properties and increasing the anti-stress relaxation properties. The incremental strength increase is due to the Orowan strengthening from the nanoscale Ni2Si and Ni3Al precipitates. As compared with the Cu–6.0Ni–1.0Si–0.5Al (wt%) alloy, the ultimate tensile strength of the designed Cu–6.0Ni–1.0Si–0.5Al–0.15Mg (wt%) alloy increases from 983.9 to 1095.7 MPa, and the electrical conductivity decreases from 27.1 to 26.6% IACS, respectively. The stress relaxation rates of the designed Cu–6.0Ni–1.0Si–0.5Al–0.15Mg alloy are 4.05% at 25 °C, 6.62% at 100 °C and 9.74% at 200 °C after having been loaded for 100 h, respectively. Magnesium significantly promotes nucleation during precipitation and maintains small precipitate size.

Influence of intermediate rolling on mechanical behavior of modified 9Cr-1Mo steel

In the present work, effect of normalizing (950–1050 °C) and tempering (650–700 °C) has been investigated to study the microstructure and mechanical properties of modified 9Cr-1Mo steel (G91) at room temperature (RT) and 650 °C. To improve the high temperature mechanical properties, hot rolling has been done at optimized normalizing and tempering conditions. Microstructural observations show that an increase in tempering temperature leads to decrease in mechanical strength both at RT and 650 °C due to the growth of M23C6 particles, formation of sub-grain boundaries and recovery. G91 steel rolled at 550 °C and tempered at 650 °C is found to have best combination of strength and fracture toughness at RT, which may be attributed to large number of small size of precipitates and increase in dislocation density. However, specimen normalized at 1050 °C and tempered at 650 °C shows more than 70% improvement in tensile strength at 650 °C as compared to as-received condition, which shows that G91 steel is more amenable to precipitation hardening. A typical type of tensile fracture with radial cracks and axial splitting is noticed at RT and − 196 °C. The tendency and severity of such radial cracks and axial splitting are found to be affected by stress triaxiality and test temperature.

Effect of Reheating Temperature and Cooling Treatment on the Microstructure, Texture, and Impact Transition Behavior of Heat-Treated Naval Grade HSLA Steel

In order to achieve the desired mechanical properties [YS > 390 MPa, total elongation >16 pct and Charpy impact toughness of 78 J at 213 K (−60 °C)] for naval application, samples from a low-carbon microalloyed steel have been subjected to different austenitization (1223 K to 1523 K) (950 °C to 1250 °C) and cooling treatments (furnace, air, or water cooling). The as-rolled steel and the sample air cooled from 1223 K (950 °C) could only achieve the required tensile properties, while the sample furnace cooled from 1223 K (950 °C) showed the best Charpy impact properties. Water quenching from 1223 K (950 °C) certainly contributed to the strength but affected the impact toughness. Overall, predominantly ferrite matrix with fine effective grain size and intense gamma-fiber texture was found to be beneficial for impact toughness as well as impact transition behavior. Small size and fraction of precipitates (like TiN, Nb, and V carbonitrides) eliminated the possibility of particle-controlled crack propagation and grain size-controlled crack propagation led to cleavage fracture. A simplified analytical approach has been used to explain the difference in impact transition behavior of the investigated samples.

Effect of initial temper on the creep behavior of precipitation–hardened WE43 alloy

The effect of initial tempers with different characteristics of precipitates and contents of supersaturated solute atoms, including the homogenized, peak–aged and over–aged conditions, on the tensile creep behavior of WE43 alloy has been investigated at 523K. Results show that the peak–aged alloy at 523K obtained superior creep resistance than the homogenized, peak–aged at 498K and over–aged at 523K alloy. A uniform dispersion of β precipitates was dynamically formed within steady–stage creep microstructure of the homogenized WE43 alloy after creep deformation of 200h. It is found that the precipitate size and distribution is similar with the alloy aged equal time without the applied stress. In addition, the WE43 alloy in all tempers obtains similar precipitate size and distribution in their steady–stage creep microstructures. Therefore, it is inferred that the various initial tempers mainly affect the primary creep stage. Furthermore, numerous dislocations were detected between precipitates and the stress exponent n is 4.5, which is close to 5. Thereby, dislocation climb is suggested to be the creep mechanism. The reason for the peak–aged alloy at 523K obtained superior creep resistance is that the initial uniform dispersion of β″ and β′ precipitates have smaller precipitate size and higher precipitate density than that of the homogenized and over–aged alloy, which are more effectively to hider dislocation climb. However, a deterioration of creep resistance was occurred in the peak−aged alloy at 498K due to precipitate recovery when crept at 523K. As a consequence, WE43 alloy in peak–aged temper at 523K achieves the highest creep resistance.

Precipitation in a Nanograined 7475 Aluminium Alloy—Processing, Properties and Nanoanalysis

The precipitation phenomena in 7475 nanograined aluminium alloy was analysed by means of microhardness measurements, small angle X‐ray scattering and electron microscopy. The nanograined samples were obtained by the hydrostatic extrusion of solution annealed and water quenched samples. It has been established that low temperature ageing causes precipitation processes to occur. However, the precipitation phenomena in nanograined materials proceed differently to those in micrograined materials. Moreover, the particle strengthening is limited by enhanced grain boundary precipitation which does not contribute to an increase in strength (when dislocation slip is the dominant deformation mechanism) and by the smaller size of precipitates.

Evaluation of the effect of crude extract purity and pure paclitaxel content on the increased surface area fractional precipitation process for the purification of paclitaxel

This study evaluated the effect of crude extract purity and pure paclitaxel content on the behavior in terms of purity, yield, fractional precipitation time, and precipitate shape and size of fractional precipitation in the increased surface area fractional precipitation process for the purification of paclitaxel. With increased pure paclitaxel content and crude extract purity, the purity and yield of paclitaxel were improved and the fractional precipitation time was reduced. Regardless of changes in crude extract purity and pure paclitaxel content, it was possible to obtain a small paclitaxel precipitate size by hindering the growth of precipitate particles using an ion exchange resin to increase the surface area. In addition, according to the type of surface-area increasing substance used, precipitate size and shape differed because of a differing affinity for the paclitaxel particles. The lower the crude extract purity and pure paclitaxel content, the higher the yield and the improvement in purity in the process of increased surface area fractional precipitation, with a greater effect on the decrease in paclitaxel particle size.

Effect of aging on the superelastic response of a single crystalline FeNiCoAlTa shape memory alloy

This study reports the effect of selected aging treatments on the superelastic response of 〈1 0 0〉-oriented Fe–28% Ni–17% Co–11.5% Al–2.5% Ta (at.%) single crystals in tension. A smaller precipitate size and lower precipitate fraction of the samples aged at 700 °C for 7 h leads to lower transformation temperatures, a wide temperature range for superelasticity and higher transformation stress levels as compared to the samples aged at 600 °C for 90 h. Both aging conditions results in fully recoverable transformation strain of 3.4% in tension.

The Microstructure and Creep Strength of 403Nb Martensitic Heat-Resistant Steel

The microstructure and creep strength of 403Nb martensitic heat-resistant steel after tempering at 650°C, 680°C and 730°C were analyzed by SEM and TEM. Microstructural observation shows that 403Nb steel tempered at 650°C provides a finer lath structure and a smaller size of precipitates than that tempered at 680°C or 730°C. The dislocation density is also higher in 403Nb steel tempered at 650°C. The creep rupture strength of 403Nb steel tempered at 650°C is the highest among those tempered. The main creep strengthening mechanisms in 403Nb steel usually include precipitates hardening, dislocation hardening and sub-grain boundary hardening.

Precipitation Sequence and Kinetics in an Fe-Si-Ti Alloy

The Fe-Si-Ti system is known to show nanoscale precipitation of the Fe2SiTi Heusler phase with potentially high volume fraction (~4%), very high density and a size ranging from 1 to 20nm after artificial aging. The strong hardening potential of these precipitates make these steels candidates for automotive applications; however no understanding of the precipitation sequence (competition with other phases) nor the precipitation kinetics are available. The present study presents a quantitative study of the precipitation kinetics (size, volume fraction and number density) in a wide temperature range (450-800°C), realised by coupling systematically Small Angle Neutron Scattering (SANS), Transmission Electron microscopy (TEM) and Tomographic Atom Probe (TAP). Tensile tests were also carried out so as to determine the microstructure/properties relationships. Along the complete temperature range, it is shown that a compromise between time for precipitation and small precipitate sizes can be reached around 550°C. At this intermediate temperature, precipitation is shown to occur in two steps, linked with a second nucleation process after nucleation &amp; growth of the first family of Fe2SiTi has been completed. This second precipitation step results in a temporary decrease in precipitate size and an increase in hardness. The nature of these precipitates is discussed in view of the TEM and TAP observations.

Precipitation in Solution-Treated Al-4wt%Cu under Cyclic Strain

Solution-treated Al-4wt%Cu was strain-cycled at ambient temperature and above and the precipitation behavior investigated by TEM. In the temperature range 100°C to 200°C precipitation of Θ´´ appears to have been suppressed and precipitation of Θ´ promoted via cyclic strain. Anomalously rapid growth of precipitates appears to have been facilitated by a vacancy super-saturation generated by dislocation motion, with a diminishing effect observed at higher temperatures due to the faster recovery of non-equilibrium vacancy concentrations. Θ´ precipitates generated under cyclic strain are considerably smaller and more finely dispersed than those typically produced via quench-aging due to their heterogeneous nucleation on dislocations, and possess a low aspect ratio and rounded edges of the broad faces due to the introduction of ledges into the growing precipitates by dislocation cutting. Frequency effects indicate that dislocation motion, rather than the extremely small precipitate size, is responsible for the observed reduction in aspect ratio. Accelerated formation of grain boundary precipitates appears partially responsible for rapid inter-granular fatigue failure following cycling at elevated temperatures, producing fatigue striations and ductile dimples coexistent on the fracture surface.

Influence of nanostructuring and heterogeneous nucleation on the thermoelectric figure of merit in AgSbTe2

In some cases, nanoscale microstructures improve thermoelectric efficiency, but this phenomenon has rarely been studied systematically for precipitates in bulk materials. We quantified the influence of nanostructuring on the thermoelectric figure of merit (zT) by embedding Sb2Te3 inclusions, from nanometer to micron sizes, in an Sb-rich AgSbTe2 matrix through solid-state precipitation. Nucleation/growth and coarsening regimes of precipitate formation had a clear effect on transport properties, which could be understood using the effective medium theory of a two-phase composite. The majority of precipitates nucleated heterogeneously at grain boundaries and at planar defects found in the matrix phase, forming a complex interconnected network. This heterogeneous nucleation causes the precipitate/matrix system to follow effective medium theory even at small precipitate sizes, thus lowering the figure of merit. Therefore, heterogeneous nucleation is a major obstacle to efficiency improvement using nanoscale precipitates in bulk thermoelectrics.

Study on the Origins of Surface Cracks at Room Temperature in Fe-Cr-Al Alloy Billets

Metallurgical and mechanical experiments were performed to explain unexpected surface cracks encountered in fabricating ground rolled-billet of Fe-Cr-Al alloys at room temperature. The toughness of these alloys containing between 220 and 236 ppm (C+N) has been assessed using notched-bar impact tests. According to our results, with a larger grain size, a higher interstitial content of (C+N) or a smaller size of precipitates, ductile to brittle temperature(DBTT) increased and absorbed energy decreased at room temperature. These results suggest that the surface cracks at room temperature stem from a poor resistance to brittle fracture, due to dislocation movement by the finely dispersed carbides within grains under the condition of higher (C+N) content.